The investigation of all-solid-state sodium–sulfur batteries (ASSSBs) is still in its early stage, where the intermediates and mechanism of the complex 16-electron conversion reaction of the sulfur cathode remain unclear. Herein, this study presents a comprehensive investigation of the sulfur reaction mechanism in ASSSBs by combining electrochemical measurements, ex situ synchrotron X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and first-principles calculations. This work, for the first time, proved that the sulfur cathode undergoes an intrinsic three-step solid–solid redox reaction following the thermodynamic principle under the extreme low rate (C-rates ≤ C/100) or at high temperature (≥ 90 °C), where S₈ is first reduced to long-chain polysulfides (Na₂S₅ and Na₂S₄), then to Na₂S₂, and finally to Na₂S, resulting in a three-plateau voltage profile. However, under conventional battery test conditions, i.e., temperatures ≤60 °C and C-rates ≥C/20, the Na₂S₂ phase is bypassed due to kinetic limitations, leading to a direct conversion from Na₂S₄ to Na₂S, resulting in the commonly observed two-plateau voltage profile. First-principles calculations reveal that the formation energy of Na₂S₂ is only 4 meV per atom lower than the two-phase equilibrium of Na₂S₄ and Na₂S, explaining its absence under kinetics-limited conditions. This work clarified the thermodynamic and kinetics-limited pathways of the 16-electron conversion reaction of the sulfur cathode in ASSSBs, providing valuable insights into the solid-state sodium–sulfur reaction mechanisms.

中文翻译：

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全固态钠电池中的三步热力学与两步动力学限制硫反应


全固态钠硫电池 （ASSSB） 的研究仍处于早期阶段，硫阴极的复杂 16 电子转换反应的中间体和机理仍不清楚。在此，本研究通过结合电化学测量、非原位同步加速器 X 射线吸收光谱 （XAS）、原位拉曼光谱和第一性原理计算，全面研究了 ASSSB 中的硫反应机理。这项工作首次证明，在极低速率（C-rate ≤ C/100）或高温（≥ 90 °C）下，硫阴极按照热力学原理进行本征的三步固-固氧化还原反应，其中 S₈ 首先被还原成长链多硫化物（Na₂S₅ 和 Na₂S₄）， 然后是 Na₂S₂，最后是 Na₂S，从而产生三平台电压曲线。然而，在常规电池测试条件下，即温度 ≤60 °C 和 C 倍率 ≥C/20），由于动力学限制，Na₂S₂ 相被旁路，导致从 Na₂S₄ 直接转换为 Na₂S，导致通常观察到的双平台电压曲线。 第一性原理计算表明，Na₂S₂ 的形成能每原子仅比 Na₂S₄ 和 Na₂S 的两相平衡低 4 meV，这解释了它在动力学受限条件下的缺失。这项工作阐明了 ASSSBs 中硫阴极 16 电子转化反应的热力学和动力学限制途径，为固态钠-硫反应机理提供了有价值的见解。